The present invention relates to a prosthesis comprising a flexible portion and at least one less flexible portion. In addition to that, the invention relates to a prosthesis consisting of a fibre-reinforced hydrogel. The invention further relates to the use of the prosthesis and to a method of manufacturing the prosthesis. The invention furthermore relates to a method of preparing the flexible portion for a prosthesis. Finally, the invention relates to a fibre material apparently intended for use in the prosthesis.
Prostheses for intervertebral discs must meet a number of requirements (Eijkelkamp et al., The International Journal of Artificial Organs, 2001, 21(5), 311-321), inter alia a correct geometry for an optimum adhesion and an optimum distribution of the pressure in relation to the adjacent vertebrae, in addition to that stiffness, in order to obtain a proper shock-absorbing action, and a swelling behaviour comparable to that of the natural intervertebral disc.
US Patent Application 2003/0045939 relates to an artificial intervertebral disc, comprising two end plates, which bound a hollow space, which is filled with an elastically and/or plastically deformable nucleus, with the hollow space being enclosed by a tubular fibre ring, made up out of for example polyamide, polyimide or polyethylene terephthalate.
U.S. Pat. No. 6,533,817 discloses a prosthetic disc nucleus apparatus, comprising a prosthetic disc nucleus for implantation, including a hydrogel core, configured to hydrate from a dehydrate state following implantation, and a retainer.
International application WO 02/17824 relates to an intervertebral disc nucleus implant, comprising a load bearing elastic body, surrounded by a resorbable shell, comprised of for example fibrin, gelatin and polymers, such as polyethylene oxide, polyethylene glycol and polyvinyl alcohol.
European patent application 0 346 129 relates to an invertebral disc spacer, comprising a central core of biocompatible elastomer and laminae wrapping said central core comprised of strips of sheets of a reinforcing fiber embedded in a biocompatible elastomer and endplates.
U.S. Pat. No. 5,047,055 discloses a prosthetic nudes for a vertebral disc made of a biocompatible hydrogel material.
The natural intervertebral disc (discus intervertebralis) is a cartilaginous disc which connects the vertebrae. A disc is built up of a gelatinous core (nucleus pulposus) enveloped in a fibrous ring (annulus fibrosus) (White et al., Clinical biomechanics of the spine, J.B. Lippencott Company, Philadelphia, 1978)). The nucleus and the annulus contain rigid collagen fibres, which are intertwined with proteoglycan chains. Said proteoglycans contain fixed, strongly negatively charged side chains (glycosaminoglycans), which interact with ions from the environment, as a result of which water is attracted by the disc. On account of the high concentration of proteoglycans, the nucleus contains 85-95% water, whereas the annulus, which comprises a relatively great deal of collagen fibres and less proteoglycans, contains 70-85% water. As a result of this specific composition, the disc allows movement of the vertebrae relative to each other and, in addition, has a shock-absorbing function. The two adjacent vertebrae comprise end plates consisting of hyalin (“vitreous”) cartilage, which serves as a transition zone between the soft intervertebral disc and the hard vertebrae.
In the case of back trouble associated with degeneration of the disc, such as a serious hernia nucleus pulposus, surgery may be necessary. In some cases part of the disc may be saved, but in serious cases the disc must be replaced in its entirety. To restore the function of the vertebral column, a prosthesis must be implanted which takes over the mechanical function of the disc both as regards its mechanical stiffness and as regards its swelling behaviour.
Examples of commercially available prostheses for replacing intervertebral discs include: a rubber core covered by titanium end plates (Acroflex®) and a polyethylene core covered by cobalt/chromium end plates (Charité®); Prostheses intended for replacing the nucleus of an intervertebral disc, which are made of hydrogel materials are known, inter alia from U.S. Pat. Nos. 5,674,295; 6,402,784 and 5,047,055, in which the annulus is filled with a hydrogel having swelling characteristics, which may or may not be surrounded by a membrane.
A fibre-reinforced hydrogel material is known from “Composite hydrogels for implants”: L. Ambrosio & Co., Proceedings of the institution of mechanical engineers, part H, Journal of Engineering in Medicine (1998), 212 (2), 93-9, Ref. 24. Use is made therein of bundles of non-absorbent polyethylene terephthalate fibres, which are incorporated in the polymerised hydrogel. For a number of uses, the material that is obtained therewith is stiff in comparison with the characteristic stiffness of the soft biological tissues that it aims to replace.
Young et al. (Biomaterials 1998, 19, 175-1752) describes a material with a base of Lycra (brand) fibre-reinforced poly-2-hydroxy ethyl methacrylate (pHEMA). Knitted or woven fibre structures are provided in a hydrogel. The fibre-reinforced hydrogel contains about 1% fibre and may be used as artificial skin.